Rework Installation and remove Quick Start tab.

Rework Installation and remove Quick Start tab.

The Quick Start tab turned out to be used a lot but not explaining what its
functionality actually does, leading to various amount of confusion. The Quick
Start tab and its functionality have been completely removed.

As replacement the reworked Installation tab now includes both the entries from
the old Installation tab (Bootloader and Rockbox) and the Extras tab (Fonts,
Themes, Game files). Each of the items can be enabled or disabled individually,
and the selection is saved in the configuration. The only exception is the
bootloader option, since installing the bootloader is only needed once. To help
with this the bootloader checkbox is automatically enabled if no Rockbox
installation is found, and disabled if one is found. While it would be nicer to
check if the bootloader is actually installed this is not possible for various
players so the implementation simply relies on a Rockbox installation.

This should also make it much easier to update an existing installation.

Current limitations:
- the selected themes are not saved.
- it is not possible to detect if the target has the plugins that require
  additional game files prior to installation. Thus the "Game files" option is
  available for all targets but simply skipped if the plugins are not found.

Change-Id: I1929bb7045e382fcbba431cca057d3121607d3a9